PROJECT SUMMARY Immune checkpoint blockade (ICB) is an immunotherapy that is revolutionizing cancer treatment, but is effective in a minority of patients. Across many cancer types, this can largely be ascribed to an insufficient number or function of tumor infiltrating T cells positioned for reactivation by ICB antibodies. Therefore, there is a critical need for strategies to increase tumor immunogenicity that results in a greater number of patients that benefit from immunotherapy. Our long-term research goal is to improve responses to immunotherapy through the molecular engineering of materials that harness endogenous mechanisms of antitumor innate immunity. To that end, we have developed STING-activating nanoparticles (STING-NPs) ? a new class of endosome- destabilizing polymer vesicles (polymersomes) that enhance the cytosolic delivery of cyclic dinucleotide (CDN) agonists of the stimulator of interferon genes (STING) pathway. CDNs have poor drug-like properties and therefore suffer from poor cellular targeting, rapid clearance, and inefficient transport to the cytosol where STING is localized. This has restricted clinical evaluation of CDNs to local, intratumoral administration, which is not feasible for many cancer patients with advanced disseminated disease. STING-NPs enhance the potency of CDNs by several orders of magnitude, resulting in increased tumor immunogenicity, inhibition of tumor growth, and improved response to ICB. Our objective in this R01 application is to further expand the utility and therapeutic window of STING-NPs by 1) optimizing their properties for safe and effective systemic administration via an intravenous route, and 2) designing new combination therapies that leverage their immunopharmacological properties to improve immunotherapy responses in melanoma models that are resistant to ICB. We will accomplish this through the following Specific Aims. First, we will re-engineer the polymersome corona to optimize the pharmacokinetics and biodistribution profile of intravenously administered STING-NP to achieve maximal CDN delivery and STING activation in the tumor microenvironment. Second, we will synthesize a new class of modified CDNs that are structurally optimized for increased incorporation and retention into STING-NPs, and will investigate the effect of CDN structure, loading, and stability on immunostimulatory activity and therapeutic efficacy. Third, we will develop rationally designed and clinically relevant chemo- and immunotherapy combinations that target mechanisms of resistance to STING agonists that we have recently identified. Overall, these studies will advance STING-NPs as a platform for increasing tumor immunogenicity and improving outcomes of immunotherapy. In doing so, these investigations will also advance our understanding of relationships between nanocarrier properties, pharmacological behavior, antitumor immunity, therapeutic activity, and toxicity with potential to inform design criteria that are broadly applicable to STING and other innate immune agonists.